Modern televisions employ many types of voltage regulators in order to generate various power supplies within the television itself. These off-the-shelf power supplies have characteristics that are known and desired vis-à-vis the ways that they perform and interact with other components within the television. Television manufacturers are comfortable with the regulators that they have employed in the past, and can be reluctant to change out this critical part.
Often, these voltage regulators rely on a resistor divider feedback signal in order to regulate their output voltages, as in FIGS. 1, 2, and 3. (These figures show the power consumer as the simple resistor RLoad, but the power consumption may be much more complex.) In circuits such as these, as power consumption increases the converter's output voltage naturally falls, the feedback voltage falling along with it. As the output voltage and the feedback voltage fall, the discrepancy in the constant comparison between the feedback voltage and a known voltage within the television or the voltage regulator itself reveals the increased power demand, which in turn causes the power supply to increase its power output. The voltage rises, the feedback voltage rises, and the system heads in the direction towards equilibrium. The process works conversely as power consumption decreases.
Consider the simple case illustrated in FIG. 4. The simplified display illustrated in FIG. 4 consists of four strings of ten LEDs per string, each string capable of being switched on and off independently of the others. Each string also contains a current sink, ensuring that each illuminated string receives the same amount of current as each other string. This ensures that each illuminated LED produces the same lighting both intensity and color—as all of the others.
Each illuminated LED requires a forward voltage of approximately 3.5 volts, and the current sink requires 1.2 volts in order to operate. Allowing for the vagaries inherent in the LED manufacturing process, each serially connected string requires about 36.2 volts for the 10 LEDs and the current sink. Because the voltage output is (in this case) fixed, in order to ensure sufficient voltage for the operation of the current sinks given the variableness of the LEDs, it would be preferable to allocate about 40 volts.
The typical current that would be desired across the feedback circuitry would be 100 microamps, implying a total resistance (R 1+R2) of 400K ohms. If the feedback voltage that the 40 volt regulator requires is 2.4 volts, we'd use resistors of 376,000 ohms and 24,000 ohms to divide the desired 40.0 volt output into the required 2.4 volts. As the strings switch on and off, the power required from the regulator goes up and down as the regulator keeps the LEDs lit.
There are some real life problems with the way that this circuit accomplishes the task of keeping the lights on. For example, the desired output voltage may not be 40.0 volts. Consider the “average” string of 10 LEDs with the “average” total forward voltage of 35.0 volts. Combined with the 1.2 required voltage drop across the current sink, the total required voltage is only 36.2 volts. With a 40.0 volt supply, all of the extra 3.8 volts worth of power is wasteful (and problematic) heat, dissipated in this example across the current sink. By considering a “worst case LED scenario” rather than an “actual requirement” scenario, excess power is generated and dissipated.
In addition, measuring the voltage at the “top” of the strings is not optimal—a better method would be to measure the feedback voltage above the current sinks as is pictured in FIG. 5, not above the LEDs as is implicit in FIG. 4. Measuring the voltage across the current sinks, where the excess voltage “accumulates,” is a better way to determine the required voltage output from the regulator—the circuit should optimally ensure that there's enough voltage (1.2 volts in this example) across each current sink, not that the LED string sees a fixed voltage. One complication with this methodology is that the circuit needs to know which current sinks are on and which are off at any particular time, as it should only ensure adequate voltage across the “on” current sinks.
And finally, there may be quite a few strings of LEDs, making it difficult to use one integrated circuit to perform the “minimum voltage” comparison. Though FIGS. 4 and 5 show ten strings of LEDs, a typical large television might have one hundred or more strings. It would be preferable to have a solution that scales across a large number of strings, a solution where that comprises a number of control and comparison chips that are linked together rather than one extraordinarily large comparison chip.
What is needed is a method for adapting these legacy voltage regulators for use in systems with variable voltage requirements that must be measured in a number of different places within the circuitry.